1. Technical Field
The invention relates to a method for fabricating an optical cable comprising at least one optical fiber surrounded by a tubular sheath, wherein an inner layer and an outer layer of the tubular sheath are extruded together around the optical fiber in a single operating step.
2. Description of the Prior Art
The attenuation of optical fibers increases when the fibers are stretched, bent and/or twisted. Moreover, fibers can only tolerate a slight elongation before breaking and can thus be damaged at comparatively low mechanical stress. Optical fibers can therefore be employed successfully only if external influences are minimized, which is accomplished by surrounding the fibers with a suitable sheath. Conventionally, one or several optical fibers are placed inside a tubular sheath made of a plastic tube. Tubular sheaths of this type must have sufficient mechanical strength and good aging characteristics, while still providing adequate flexibility. In order to satisfy these requirements, dual layer plastic tubes are commonly employed which consist of an inner layer with a very low friction coefficient and an outer layer which is mechanically stable and resists aging. Such dual layer tubular sheaths are usually fabricated in a continuous process using two consecutive extruders. It is further known to simplify the manufacturing process by using co-extrusion with a single extrusion machine, so that the two layers of a dual layer sheath which loosely surrounds one or more optical fibers can be fabricated in a single operating step. To prevent water or other fluids from migrating along the tubular sheath if an optical buffer tube or multi-fiber buffer tube is damaged, the tubular sheath can be filled with a suitable thixotropic substance.
Various cable designs are used for the different applications and installation methods of optical cables. For example, in telecom report 17 (1994), Vol. 4, pages 170 to 172, there is depicted an aerial cable which includes an aramide thread lapping which is applied over a central multi-fiber buffer tube formed by a tubular sheath containing a plurality of optical fibers, for relieving tensile stress to the cable, and an outer jacket. Due to the aramide thread lapping and the outer jacket, the diameter of the cable is much larger than the diameter of the multi-fiber buffer tube.